AUGUSTA, Maine — Gov. Paul LePage’s endorsement of the controversial Keystone pipeline project, which he detailed in a March 3 letter to Secretary of State John Kerry, added new fuel to a long-term debate about bringing tar sands oil through Maine.

At issue is a nearly year-old application from the TransCanada Corp. to build an oil pipeline from the Canadian border to Steele City, Neb., the permitting of which is up to President Barack Obama.

“I support that application,” wrote LePage in his letter to Kerry while arguing that denying the project could harm trade relationships between the U.S. and Canada and by extension, put Maine’s exports of goods to Canada — which LePage said amounts to more than $800 million annually — at risk.

“As a state with one of the longest borders with Canada, ensuring that the review of projects is fair and expeditious is absolutely essential for Maine’s economic future,” wrote LePage. “Adding uncertainty to major cross-border projects will only result in Canada looking to projects that do not require U.S. regulatory approval and may avoid the state of Maine. This is the wrong direction for the world’s most vibrant economic partners.”

The governor did not explicitly mention ongoing speculation among Maine environmental groups that Portland Pipe Line Corp. would seek to reverse the flow of materials through an existing South Portland-to-Montreal pipeline to bring tar sands oil to South Portland and beyond. To date, there has been no concrete proposal to do so.

“The United States and Canada must develop practical solutions to address shared environmental challenges and we are working on these issues in New England and with our counterparts in the Eastern Canadian Provinces,” wrote LePage.

Glen Brand, director of Sierra Club Maine, said Thursday that LePage’s statement concerns him and other environmentalists in Maine.

“It’s not surprising that the governor would be on the side of big oil on this issue, but what causes us more concern is that it sounds to us like he’s suggesting to bring tar sands oil through northern New England and Maine,” said Brand. “He has not explicitly said he is in favor of or opposed to bringing tar sands oil through the existing pipeline and that should cause concern for the thousands of people in Maine who would be affected by it.”

Patrick Woodcock, the director of LePage’s Office of Energy Independence, said Thursday that LePage’s letter to Kerry focused on Maine’s trade relations with Canada.

“What he’s highlighting there is that we have a huge stake in the economy with our partnership with Canada,” said Woodcock. “Whether it’s transmission lines or transportation projects, we are integrated in the Canadian economy and we should take that partnership seriously.”

LePage has not said publicly whether he supports bringing tar sands through Maine.

“The governor supports proposals for any economic development, but then you’ve got to weigh the energy benefits and environmental costs when that project is proposed,” said Woodcock.

The concept of the Montreal-to-South Portland project has been at the center of controversy for months. In November 2013, South Portland voters narrowly defeated a waterfront protection ordinance that would have essentially blocked the loading of oil sand crude into ships at South Portland’s waterfront. City officials subsequently passed a six-month moratorium on exporting oil sand crude through the city’s port facilities as a way to buy time to formulate new municipal guidelines.

Members of Maine’s congressional delegation have been proactive about the possibility of any proposals to bring tar sands through Maine. Earlier this month, Sen. Angus King, I-Maine, wrote his own letter to Kerry urging that a presidential permit and extensive environmental impact study be part of any future approval process for a project in Maine.

“My constituents have consistently expressed concern at the lack of any environmental review of a project of this nature, given that there appears to be no substantive state review process that would be triggered,” wrote King to Kerry.

Democratic U.S. Reps. Chellie Pingree and Mike Michaud made a similar request for strict environmental reviews for pipeline projects to Kerry in a February 2013 letter that was signed by 17 members of Congress.